According to SOHO
documentation, "Over time, many types of CCDs (Charge-Coupled Devices)
used to detect EUV radiation are degraded by contamination of heavy
ions from cosmic/solar particle radiation (interfering with the doping
of the chip), deposits and polymerisation of deposits on the surface
and, finally, trapped charges (again interfering with the doping of
the substrate). These forms of degradation reduce the sensitivity of
the detector. Fortunately, by heating the CCD over an extended time
period, some of these effects can be reduced."